US7249516B2 - Method of operating a resistive heat-loss pressure sensor - Google Patents

Method of operating a resistive heat-loss pressure sensor Download PDF

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US7249516B2
US7249516B2 US11/146,721 US14672105A US7249516B2 US 7249516 B2 US7249516 B2 US 7249516B2 US 14672105 A US14672105 A US 14672105A US 7249516 B2 US7249516 B2 US 7249516B2
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current
sensing element
compensating
sensing
resistive
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US20060021442A1 (en
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Michael D. Borenstein
Paul C. Arnold
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Barclays Bank PLC
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Brooks Automation Inc
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Priority claimed from US10/900,504 external-priority patent/US20060021444A1/en
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Assigned to HELIX TECHNOLOGY CORPORATION reassignment HELIX TECHNOLOGY CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ARNOLD, PAUL C., BORENSTEIN, MICHAEL D.
Priority to US11/146,721 priority Critical patent/US7249516B2/en
Priority to KR1020077004724A priority patent/KR20070085218A/ko
Priority to AT05772352T priority patent/ATE555373T1/de
Priority to PCT/US2005/025394 priority patent/WO2006020196A1/en
Priority to JP2007523626A priority patent/JP4809837B2/ja
Priority to EP05772352A priority patent/EP1771711B1/en
Assigned to BROOKS AUTOMATION, INC. reassignment BROOKS AUTOMATION, INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HELIX TECHNOLOGY CORPORATION
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Publication of US7249516B2 publication Critical patent/US7249516B2/en
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Assigned to BARCLAYS BANK PLC, AS COLLATERAL AGENT reassignment BARCLAYS BANK PLC, AS COLLATERAL AGENT CORRECTIVE ASSIGNMENT TO CORRECT THE REMOVE U.S. PATENT NO.7,919,646 PREVIOUSLY RECORDED ON REEL 048211 FRAME 0312. ASSIGNOR(S) HEREBY CONFIRMS THE PATENT SECURITY AGREEMENT (ABL). Assignors: ELECTRO SCIENTIFIC INDUSTRIES, INC., MKS INSTRUMENTS, INC., NEWPORT CORPORATION
Assigned to NEWPORT CORPORATION, MKS INSTRUMENTS, INC., ELECTRO SCIENTIFIC INDUSTRIES, INC. reassignment NEWPORT CORPORATION RELEASE BY SECURED PARTY (SEE DOCUMENT FOR DETAILS). Assignors: BARCLAYS BANK PLC
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L21/00—Vacuum gauges
    • G01L21/10—Vacuum gauges by measuring variations in the heat conductivity of the medium, the pressure of which is to be measured
    • G01L21/12—Vacuum gauges by measuring variations in the heat conductivity of the medium, the pressure of which is to be measured measuring changes in electric resistance of measuring members, e.g. of filaments; Vacuum gauges of the Pirani type
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L11/00—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00
    • G01L11/002—Measuring steady or quasi-steady pressure of a fluid or a fluent solid material by means not provided for in group G01L7/00 or G01L9/00 by thermal means, e.g. hypsometer

Definitions

  • the rate of heat transfer through a gas is a function of the gas pressure.
  • measurements of heat transfer rates from a heated sensing element can, with appropriate calibration, be used to determine the gas pressure. This principal is used in the well-known Pirani gauge.
  • Pirani gauges comprise temperature sensitive sensing and compensating resistances in separate legs of a Wheatstone bridge.
  • the compensating resistance is sized to minimize self-heating with current applied through the two resistances.
  • the resultant resistance differences with heating of the sensing resistor is indicative of pressure of the surrounding environment.
  • the sensing element and compensating element are of like dimensions, but an additional heating current is applied to the sensing element to boost its temperature. Again, the relative resistances of the sensing and compensating elements with increase in temperature of the sensing element are indicative of the pressure of the surrounding environment.
  • One implementation relies on a Wheatstone bridge, while another relies on a fixed ratio of current flow through the resistive elements under control of a feedback circuit responsive to the sensed resistances.
  • the present invention relates to an improvement to a heat-loss gauge which has the potential of providing higher performance at a reduced cost due to the ability to rely on less precise components.
  • the present system controls power to the sensing and compensating elements using asymmetrical switching techniques.
  • An electrical source is connected to switch current between the sensing element and compensating element, preferably from a common current source.
  • Current is applied to the sensing element over a longer duty cycle to heat the sensing element relative to the compensating element.
  • Measuring circuitry determines gas pressure in the environment to which the elements are exposed based on electrical response of the sensing element and the compensating element.
  • current is applied to the sensing element and compensating element at fixed duty cycles and current level is controlled.
  • the applied current is fixed and duty cycle of current to at least one of the sensing element and compensating element is controlled.
  • both the current and the duty cycles are controlled.
  • the gas pressure may be determined based on the level of heating current through the sensing element and/or the resulting voltage across the sensing element.
  • the compensating element is in series with a fixed resistive element.
  • the electrical source applies current to heat the sensing element to a temperature at which the resistance of the sensing element matches the combined resistance of the compensating element and the fixed resistive element.
  • the fixed resistive element is only in series with the compensating element, and the voltage across the compensating element and fixed resistive element is compared to a voltage across the sensing element to control the switched current.
  • the fixed resistive element is in series with both the sensing element and the compensating element, and the voltage across the fixed resistive elements is added to the voltage across the compensating element and fixed resistive element for comparison to a voltage across the sensing element and fixed resistive element.
  • FIG. 1 shows one embodiment of the present invention.
  • FIG. 2 shows the embodiment of FIG. 1 in greater detail.
  • FIG. 3 illustrates another embodiment of the invention designed to reduce thermoelectric effects using a synchronous detection technique.
  • FIG. 4 illustrates another embodiment to avoid the effects of stray resistance in connection paths.
  • FIG. 5 illustrates another embodiment that varies current duty cycle.
  • FIG. 1 is a simplified diagram of control and measuring circuitry embodying the invention.
  • the purpose of the sensor control circuit is to cause the temperature of the sensing element Rs to be maintained at a precise fixed amount above the temperature of the compensating element Rc.
  • the voltage across the sensing element and/or current through the sensing element required to do this are measured and then converted into a pressure in a manner described in U.S. Pat. Nos. 6,023,979 and 6,658,941 which are incorporated by reference in their entirety.
  • Current from the dependent current source I 1 is alternately switched through the sensing element Rs and the compensating element Rc using switch S 3 .
  • the time during each cycle that the current flows through the sensing element Rs is greater in proportion to the time that the current flows through the compensating element Rc.
  • the average power dissipated in Rs is greater than that dissipated in Rc, causing Rs to rise to a higher temperature than Rc.
  • the resistance Rs will increase to a greater amount with a given input, or will require a lesser power input to increase to a given resistance.
  • the extent to which the resistance Rs increases over the resistance Rc is readily determined by connecting a non-temperature dependent differential resistance Rd in series with Rc and driving the resistance Rs to a level at which Rs equals Rc plus Rd. The electrical input required to maintain that equality of resistances can then be used to compute pressure.
  • Alternative approaches might, for example, rely on measurements of Rs and Rc that are digitized and processed in a microprocessor without the series resistance Rd.
  • the cycle period of this process is kept much shorter than the thermal time constant of the sensor wires so that the temperatures, and therefore the resistances, of the elements do not change as the current is switched back and forth.
  • a fixed resistor Rd is inserted in series with Rc to form a sum of a temperature-dependent and a non-temperature-dependent resistance.
  • the difference V 1 ⁇ V 2 is amplified in the high-gain integrating amplifier A 1 which drives the dependent current source I 1 to the proper level to maintain the conditions of equal voltages and equal resistances.
  • the gain of amplifier A 1 is sufficiently high to keep the error between V 1 and V 2 negligible, and the time response of amplifier A 1 is slow enough to assure that current source I 1 cannot change value during the switching cycle time.
  • Current meter Is measures the sensing element current.
  • amplifier A 1 holds the current of I 1 equal for both parts of the switching cycle, causing the current through meter Is to be a steady DC level equal to that of the current of source I 1 .
  • the current measured in current meter Is is equal to the peak sensing element current Is, which is equal to the current of source I 1 .
  • the average voltage across Rs is developed across C 3 of an RC filter with a time constant somewhat longer than the cycle time of the current switching cycle.
  • the average sensing element voltage Vs and the current Is are converted to a digital format using standard A/D conversion techniques.
  • a digital processor calculates pressure as a function of Vs and Is using an algorithm that was developed using empirical 3-D surface fitting techniques as described in U.S. Pat. Nos. 6,023,979 and 6,658,941.
  • the present switched design allows for a reduction in the precision components which were used in the implementation of FIG. 7 of U.S. Pat. No. 6,658,941.
  • two current sources had precise current ratios.
  • matched dual operational amplifiers and precision resistances were used.
  • precision resistances were used to provide accurate multiplier ratios in a feedback circuit that controlled the current sources.
  • a single current source applies the current to both legs of the circuit.
  • voltages v 1 and v 2 are provided directly back to the amplifier A 1 without the need to have one divided relative to the other. Rather than precisely controlling ratios of currents and voltages, the present design relies on time ratios that are easily controlled by low-cost digital circuits.
  • a timing circuit generates digital timing signals B, C and D to guarantee that S 1 closes after the current switches to the compensating element and opens before the current switches to the sensing element, and S 2 closes after the current switches to the sensing element and opens before the current switches to the compensating element.
  • the current source I 1 is comprised of an FET Q 1 and resistors R 1 and R 2 .
  • the switch S 3 comprises FETs Q 2 and Q 3 driven by respective timing signals B and A. It was found from experimental data that a cycle frequency above 3 kHz eliminated thermal time constant issues, and frequency was chosen to be 10 kHz.
  • the switching duty cycle was set at 25% for the compensating element and 75% for the sensing element. Although duty cycles up to nearly 50% will work, a shorter duty cycle reduces undesirable self-heating of the compensating element.
  • the compensating element temperature can be kept close to the ambient envelope temperature of the device, minimizing unnecessary power dissipation and case temperature rise.
  • the power dissipation, and therefore the temperature rise of the compensating element is slightly less (about 80%) than 1/(compensator-to-sensor time ratio) 2 of the sensing element.
  • thermoelectric effects are illustrated in FIG. 3 as voltage sources V th-c and V th-s .
  • the method described above can be further improved so that these thermoelectric effects can be eliminated using a.c. synchronous detection schemes. Since current is alternately switched between the two elements, the voltage across each element can be detected during each respective cycle state. The difference between the two detected voltages developed across a given element provides a more accurate resistance and heating voltage measurement since the residual thermoelectric error voltages are present in both readings and therefore cancelled out.
  • V th-c is the undesirable thermoelectric voltage that occurs when measuring the voltage on the compensating element
  • V th-s is the undesirable thermoelectric voltage that occurs when measuring the voltage on the sensing element.
  • Instrumentation amplifiers A 2 and A 3 which have equal gains, produce an output voltage proportional to V 1 ⁇ V 4 and V 2 ⁇ V 3 , respectively.
  • the effects of V th-c and V th-s are both eliminated in the outputs of these two amplifiers.
  • the sensing element heating voltage is sensed by measuring the differential voltage V 2 ⁇ V 3 , also eliminating the effect of V th-s .
  • V th-s the differential voltage
  • the excitation current is passed through a fixed resistor, and the voltage across this fixed resistor is added to the sampled voltage from the connection to the compensating element.
  • the method illustrated in FIG. 4 is to use a “switched capacitor” technique where a floating capacitor is charged to the voltage across the fixed resistor during the longer phase of the current duty cycle.
  • this capacitor is connected in series with the sensed voltage on the compensating element during the shorter phase of the current duty cycle in order to charge the sample-and-hold capacitor for the compensating element voltage to the sum of the two voltages. This is accomplished by adding three analog switches and a capacitor to the original circuit.
  • This new method has the potential of providing higher performance in addition to reduced cost.
  • the previous DC methods are subject to thermoelectric errors that result from small temperature gradients.
  • This method has the advantage of producing higher voltage signal levels, making the thermoelectric errors smaller relative to the signal levels.
  • the compensating element can be operated at a much lower power level in proportion to that of the sensing element, reducing undesirable heat dissipation. Since this method operates in a pulsed mode, further performance improvement can be achieved by using the AC measurement technique of FIG. 4 , eliminating all thermoelectric instabilities.
  • An important advantage of increasing instrument performance is the added pressure range that can be realized.
  • the fixed resistance Rd is in series with both the sensing and compensating elements Rs and Rc.
  • the voltage across that resistor has a common level present in both signals v 2 and v 1 .
  • the circuit of FIG. 4 additionally adds the voltage across Rd to the sampled peak value of v 1 to become comparable to the prior designs.
  • the voltage across Rd is stored on C 4 by closing switches S 6 and S 7 .
  • the switches S 6 and S 7 are open so that the capacitor C 4 is connected in series with the circuit from v 1 to S 1 .
  • the voltage stored on C 1 is the sum of the peak value of v 1 and V C4 .
  • FIG. 5 corresponds to a modification of the circuit of FIG. 1 , but this approach can be applied to any of the prior embodiments.
  • the switch timing control STC which may be a primarily digital circuit, responds to the voltages v 1 and v 2 to determine the appropriate time-lengths during which switch S 3 is connected to the sensing element Rs and to the compensating element Rc.
  • the current through current source I 1 may be held constant as the relative lengths of the pulses applied to Rs and Rc are controlled.
  • the time-length of the current pulse to the sensing element Rs is the control parameter that preferentially heats the sensor to a specified resistance.
  • the magnitude of the current pulses would be fixed and would be the same for both Rs and Rc.
  • the time-length of the pulse to Rc would typically be fixed, but could be variable.
  • both the level of current through the current source is controlled and the lengths of the pulses to Rs and Rc are controlled.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Measuring Fluid Pressure (AREA)
  • Measurement Of Force In General (AREA)
US11/146,721 2004-07-28 2005-06-07 Method of operating a resistive heat-loss pressure sensor Active US7249516B2 (en)

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Application Number Priority Date Filing Date Title
US11/146,721 US7249516B2 (en) 2004-07-28 2005-06-07 Method of operating a resistive heat-loss pressure sensor
EP05772352A EP1771711B1 (en) 2004-07-28 2005-07-19 Method of operating a resistive heat-loss pressure sensor
PCT/US2005/025394 WO2006020196A1 (en) 2004-07-28 2005-07-19 Method of operating a resistive heat-loss pressure sensor
AT05772352T ATE555373T1 (de) 2004-07-28 2005-07-19 Verfahren zum betrieb eines widerstands- wärmeverlust-drucksensors
KR1020077004724A KR20070085218A (ko) 2004-07-28 2005-07-19 저항성 열손실 압력 센서의 작동 방법
JP2007523626A JP4809837B2 (ja) 2004-07-28 2005-07-19 抵抗による熱損失式圧力センサの動作方法

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US7081905A 2005-03-01 2005-03-01
US11/146,721 US7249516B2 (en) 2004-07-28 2005-06-07 Method of operating a resistive heat-loss pressure sensor

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US7360439B2 (en) * 2006-02-24 2008-04-22 Fujitsu Limited Sample resistance measurement device
WO2019217230A1 (en) 2018-05-09 2019-11-14 Mks Instruments, Inc. Method and apparatus for partial pressure detection

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US7331237B2 (en) * 2006-02-01 2008-02-19 Brooks Automation, Inc. Technique for improving Pirani gauge temperature compensation over its full pressure range

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US1873984A (en) 1928-09-14 1932-08-30 Sieber Fritz Indicating device
US2938387A (en) 1956-12-10 1960-05-31 Cons Vacuum Corp Automatic control circuit
US3066537A (en) 1958-07-24 1962-12-04 New York Air Brake Co Pressure gauge
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US3794885A (en) 1971-11-10 1974-02-26 Tokyo Shibaura Electric Co Static type electric circuit breaker
US4159428A (en) 1976-10-11 1979-06-26 Antonov Boris M Method of dividing direct current among parallel circuits and device for effecting same
US4106350A (en) 1977-08-29 1978-08-15 Morris Richard T Thin wire pressure sensor
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US20060021442A1 (en) 2006-02-02
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